The present invention relates to a process for producing SC/LWC qualities, by upgrading TMP intended for newsprint.
The general technique presently used for the production of high quality TMP (thermomechanical pulp) for SC/LWC (super calendared/lightweight coated) qualities is to subject the pulp to a two-stage refining process in the main processing line. These refining processes require a relatively high-energy input level and subsequent screening in two stages for selective separation of a long fiber fraction. This fraction is processed at a high specific energy input level in two high consistency-refining stages, with renewed screening between the stages. The fiber length of the high consistency refined reject fraction is optionally controlled with a finishing refining stage with low pulp consistency.
One of the problems which exist with the use of TMP newsprint qualities as SC/LWC qualities, is that the fiber distribution is unfavorable with an average fiber length which is too high, and has a proportion of coarse rigid fibers of low binding strength which is also too high.
At present, the upgrading of TMP is often effected in plants in which only a partial quantity of the TMP-production is upgraded to SC/LWC. The upgrading is effected by high consistency refining of the finished newsprint pulp or, after screening, of its long fiber fraction in one or more stages. The result of doing so is often that the pulp gets an unfavorable fiber distribution, with an average fiber length which is too high, and a proportion of rigidfibers which is also too high, with accompanying impaired surface properties of the end product, because the screening has not been sufficiently selective at prevailing screening conditions, particularly at consistencies of more than 2%.
In accordance with the present invention, these and other problems in the prior art have been solved by the invention of a process for the processing of a low consistency newsprint pulp having a consistency of less than 5% comprising refining the low consistency newsprint pulp in a first refiner to provide a processed low consistency pulp having a consistency of greater than 2%, screening the processed low consistency pulp with a slotted screen basket having a slot width of from 0.05 to 0.15 mm to provide a primary accept portion and a primary reject portion, with the withdrawal of the primary reject portion being greater than 50%, withdrawing the primary accept portion for further processing, dewatering the primary reject portion to provide a high consistency pulp, refining the high consistency pulp in a second refiner to provide a processed high consistency pulp, screening the processed high consistency pulp with a slotted screen basket having a slot width of less than 0.15 mm to provide a secondary accept portion and a secondary reject portion, and combining the secondary accept portion with the primary accept portion for the further processing. In a preferred embodiment, the low consistency newsprint pulp has a consistency of from 2% to 5%, and the refining of the low consistency newsprint pulp in the first refiner includes a specific energy input level of from 50 to 200 kWh/t.
In accordance with one embodiment of the process of the present invention, the high consistency pulp has a consistency of from 20% to 40%, and the refining of the high consistency pulp in the second refiner includes a specific energy input level of from 600 to 1400 kWh/t.
In accordance with another embodiment of the process of the present invention, the refining of the low consistency newsprint pulp in the first refiner provides a processed low consistency pulp having a consistency of greater than 2.5%, and the screening of the processed low consistency pulp utilizes a slotted screen basket having a slot width of from 0.05 to 0.12 mm.
In accordance with another embodiment of the process of the present invention, the screening of the processed low consistency pulp includes a withdrawal of the primary reject portion of greater than 55%.
The above mentioned problems have been solved in accordance with the present inventive process, by placing a low consistency refiner (LC-refiner) as a first upgrading stage in direct connection to a fractionating screen room, that includes equipment for slot screening, with a narrow slot width, of the stiff long fiber fraction. This upgrading stage, due to its position, regulates the fiber length, the fiber flexibility and the shive content of the pulp, in a particularly favorable manner for the screening process. Thus, in the inventive process for producing upgraded TMP to an SC/LWC quality, the newsprint pulp with a consistency of less than 5% is pumped to a LC-refiner, where the average fiber length is reduced by 10% to 25%, and subsequent transfer of the pulp with a consistency of more than 2% to a primary screen, which has a slotted screen basket with a slot width of from 0.05 to 0.15 mm. The pulp is divided into primary accept and primary reject portion, whereby the reject withdrawal exceeds 50%. The primary accept portion is transferred to the main line for further processing. The primary reject portion is dewatered prior to again being refined in at least one high consistency refiner (HC refiner). The refined primary reject portion is then transferred to a secondary screen which has a slotted screening basket with a slot width of from 0.05 to 0.15 mm, and is divided into a secondary accept portion and a secondary reject portion. The secondary accept portion is returned to the main line and mixed with the primary accept portion from the primary screen.